The time machine / H.G. Wells.

A Victorian scientist develops a time machine and travels to the year 802,171 AD. There he finds the meek, child-like Eloi who live in fear of the underground-dwelling Morlocks. When his time machine goes missing, the Traveller faces a fight to enter the Morlocks' domain and return to his own time. "The time machine" remains one of the cornerstones of science-fiction literature and has proved hugely influential.